The advent of lightwave communications has stimulated an intensive development of methods for assembling photonic devices. Photonic devices are those having both optical and electrical attributes, such as lasers and photodetectors. A complex lightwave communications system may require many photonics devices, each of which must be accurately and dependably mounted in alignment with an optical element such as an optical fiber or a lens. It is important to automate as much as possible the process for assembling such devices and to reduce the operator skill needed for obtaining the requisite alignments.
As described, for example, in the papers, "Self-Alignment Capability of Controlled-Collapse Chip Joining," L. S. Goldmann, Proceedings of 22nd Electronic Components Conference, 1972, pp. 332-339 and "A Novel Flip-Chip Interconnection Technique Using Solder Bumps for High-Speed Photoreceivers," K. Katsura et al., Journal of Lightwave Technology, Vol. 8, No. 9, September 1990, p. 1323, both hereby incorporated herein by reference, a method known as solder self-alignment can be used to obtain accurately aligned bonding and conductive interconnection of a semiconductor chip such as a photonic device to a substrate. According to the method, a set of bonding pads on the substrate is coated with solder and the semiconductor chip is mechanically placed with corresponding bonding pads overlying the substrate bonding pads. Although final registration is desired to be accurate to within one or two microns, the mechanical placement need be accurate only to within several tens of microns. The assembly is then heated to melt or "reflow" the solder, which causes the solder on each of the substrate bonding pads to gather and to wet to a matching bonding pad of the semiconductor chip. The surface tension of the molten solder element tends to force it to flow such as to distribute itself symmetrically around a vertical axis or plane. This force moves the chip, thereby accurately aligning the chip with the substrate. The chip may, for example, be a photodetector or a laser which is accurately aligned, for example, with an optical fiber bonded to the substrate.
One requirement for the solder self-alignment method is that the solder globule wet the bonding pads to which it adheres. A liquid flux is usually coated on the bonding pads for this purpose, but it is recognized that such flux can cause a number of problems. The flux may, for example, bubble during the solder process which may dislodge the chip or contaminate surfaces of the chip or optical elements to interfere with optical transmission. We have tried to avoid the use of a liquid flux by performing the soldering operation in a reducing atmosphere such as an atmosphere of carbon monoxide or hydrogen but have found that such a substitution is not satisfactory. Our studies have shown that, with such substitutions, the molten solder elements often do not from the aligned symmetrical configurations required for solder self-alignment. Accordingly, there is a continuing long-felt need for improved methods for using solder self-alignment to bond semiconductor chips to substrates.